Dichroic reversible photosensitive materials

ABSTRACT

New materials are the dichroic reversible photosensitive materials which are the crystals of an alkali metal halide in which are contained an alkali metal ion having smaller ion radius than that of the alkali metal in said alkali metal ion and one or more kinds of metal ions other than alkali metal ion which possess an ability to act as an electron trap; which have Mcenter. Said materials are useful as a reversible ultramicro fish, an optical storage element for optical memory system and the like.

United States Patent [191 Kishi et al.

[ DICHROIC REVERSIBLE PHOTOSENSITIVE MATERIALS [7 5] Inventors: Yasuo Kishi, Hirakata; Toshiaki Yokoo, Osaka; Terutoyo Imai, Neyagawa; Keiichi Kanatani, Hirakata; Takehito Yazaki, Katano. all of Japan [73] Assignee: Director of Agency of Industrial Science & Technology, Japan [22] Filed: Aug. 28, 1973 [21] Appl. No.: 392,302

[30] Foreign Application Priority Data Aug. 30. 1972 Japan 47-86608 [52] US. Cl. 96/88; 96/90 PC; 252/300 [51] Int. Cl. G03C 1/00 [58] Field of Search 96/88, 90 PC, 108;

[4 1 Sept. 30, 1975 [56] References Cited UNITED STATES PATENTS 3,580688 5/1971 Schneider 356/256 $720,926 3/1973 Schneider..... 340/173 CC Gilman et al.

Schneider .7

Primary E.\'aminer--Won H. Louie, Jr.

[5 7] ABSTRACT 10 Claims, No Drawings DICI-IROIC REVERSIBLE PHOTOSENSITIVE MATERIALS This invention relates to dichroic reversible photosensitive materials and a method for the preparation thereof. More particularly, it concerns dichroic reversible photosensitive materials which consist of the crystals of an alkali metal halide which contain an alkali metal ion having smaller ion radius than that of the alkali metal in said alkali metal halide and also one or more kinds of other metal ions than alkali metal ion which possess an ability to act as an electron trap; which have M-center, and a method for the preparation therof.

The objects of this invention'will become clear in the following explanation. i

It has been known that when crystals of an alkali metal halide suchas sodium chloride, potassium chloride or the like are irradiated with an electron beam, proton, X-ray or y-ray, or treated in an atomosphere of alkali metal steam, negative ion vacancy is generally produced in the crystals. And the negative ion vacancy is one of color centers, because it can catch an electron and absorb light at a definite wave length by which crystals are colored. An M-center is one of such color centers wherein two negative ion vacancies (F-center) stand adjacently to each other to the direction of l 10) or (1T0) and have two absorption bands and together catch an electron. That is, the M-center oriented to the direction of 1 l), e.g., that of sodium fluoride shows a maximum absorption band at 505 mp. to the light polarized to the direction of (l and an absorption band at 340 M,u. to the light polarized to the direction of (1 1 0). The band at long wave length side between two absorption bands is called as the first absorption band and one at short Wave length side as the second absorption band. M-center possesses a property of being not reoriented by the light of first absorption band but reoriented by the light of second absorption band to the direction of polarization, e.g., the M-center oriented to the direction of (1T0) is reoriented to the direction of (1 l0) by irradiation of the light at second absorption band polarized to the direction of (110). Accordingly, it will be possible to write and read an information in using such property of M-center. When an information 0 is appllied for the M-center oriented to the direction of l 10) and an information 1 for the M- center oriented to the direction of( 1T0), writing of the information 0 in crystals by the light of second absorption band polarized to the direction of (110) will be possible. And it will be also possible to erase an information O in M-center by the light polarized to the direction of l 10) and stimutaneously to write a new information 1.

Also, as numbers of M-center to be reoriented are proportional to exposing energy of writing light, the crystals having M-center will possess a capacity of an analogue record for voluntary pattern such as characters, figures and the like as well as of a digital record as mentioned above. They will, therefore, be usable for a photosensitive material as equivalent to photographic 'films. Furthermore, a great advantage to use M-center as above is that crystals having M-center become a reversible photosensitive material which is capable of recording and erasing an information repeatedly. Therefore the crystals'are promising as a memory element. In utilization of M-center, the inventors of this invention have discovered a reversible photosensitive material which consists of the crystals of an alkali metal halide intermixed with one kind of a specific metallic cation in which M-center is formed, and as a result of further studies on using a new theory developed by them, have succeeded to obtain new dichroic reversible photosensitive materials having a remarkably improved photosensitivity which consist of the crystals of an alkali metal halide intermixed with one kind of alkali metal ion having smaller ion. radius than the alkali metal in said alkali metal halide used as the base mate- I rial and one or more kinds of metal ions other than the alkali metal ion which possess an ability to act as an electron trap, which have M-center.

The alkali metal halide crystal to be used as the base material of this invention means the crystals of salts of an alkali metal such as lithium, sodium, potassium, rubidium or cesium, with a halogen such as chlorine, bromine, fluorine or iodine. The most preferred example of the alkali metal halide may be sodium fluoride. Preferred examples may be sodium chloride, sodium bromide, potassium chloride, potassium fluoride, potassium bromide, lithium fluoride, rubidium fluoride, rubidium bromide and the like. Besides, cesium fluoride, cesium bromide, sodium iodide and the like may be mentioned as other examples to be used in this invention.

The alkali metal ion in the compound producing an alkali metal ion having smaller ion radius than that of the alkali metal in the alkali metal halide means, for instance, lithium ion to sodium ion, or sodium or lithium ion to potassium ion or the like. The compound producing an alkali metal ion having smaller ion radius than that of the alkali metal in the alkali metal halide is one producing an alkali metal ion as mentioned above, preferably a halide compound of the same kind of halogen as that of the alkali metal halide used.

The metal ion in the compound producing metal ion other than the alkali metal ion which possesses an ability to act as an electron trap, means the ion derived from metals as mentioned below.

i. an alkali earth metal such as beryllium, magnesium, calcium, strontium or barium.

ii. a metal belonging to the group I B of the periodic table such as copper or silver.

iii. a metal belonging to the group II B of the periodic table such as zinc, cadmium or mercury.

iv. a metal belonging to the group III B of the periodic table such as gallium, indium or thallium.

v. a metal belonging to the group IV B of the periodic table such as lead or tin. I

vi. a transition metal such as iron, cobalt, nickel, titanium, vanadium, chronium or magnesium.

vii. a rare earth metal such as samarium or europium.

The compound producing metal ion other than alkali metal ion which possesses an ability to act as an electron trap means the compound producing the metal ions as mentioned above. Preferable one is a halide compound of the same kind of halogen as that of the alkali metal halide to be used.

Speaking of the method for preparing the object materials in this invention, an appropriate amount of alkali metal halide crystal is firstly melted, e,g., in a melting pot or bath, to which are added one kind of compound producing alkali-metal ion and one or more kinds of compounds producing metal ions and melted. Crystals are grown and isolated in accordance with a conventional method, e.g., Kyropoulos method. See, e.g.,

Color Centers In Solids, J. H. Schulman and W. D. Compton, Pergamon Press, New York (1963), pages 32-34.

The addition amount of each of the compound producing alkali metal ion and the compounds producing metal ion may be about 0.00001 10.0 molar percentage to the crystals of alkali metal halide as the base material, preferably about 0.1 2.0 molar percentage. However, even though the addition amount of each of the compounds is increased over the above figure, it will be meaningless, because an amount to be practically intermixed with the alkali metal halide crystal depends upon a coefficient of segregation of the crystals used. applying /cm- Next, M-center is formed in the resulting crystals by a conventional method. As its examples may be mentioned a method for applying, an electron beam, X-ray or 'y-ray, an electrolytic coloring method and the like. It is desired to make M-center at a concentration as high as possible, preferably a concentration of about l cm 18 cm. For example, when a comparatively weak electron beam is applied to the crystals for a long time, it can produce the crystals having a high concentration of M-center.

There are various combinations of the alkali metal halide crystal, another alkali metal ion and other metal ions as the components of the materials in this invention. Taking an example of sodium fluoride as the alkali metal halide crystal, there may be mentioned as follows.

NaF Li Ag NaF Li Pd, NaF Li Mg, NaF Li Be, NaF Li Sr, NaF Mg Cr, NaFzLi-Mg-Cr, NaFzLi-Ag-Be-Cr, NaF Li Tl Mg Ni, etc.

Also, in case of potassium chloride as the base material, there may be mentioned as follows.

KCl Nu Ag, KCl Li Ag, KCI Na Mg, KCl Li Be, KCl Li Sr, KCl Na Mg -Cr, KClzLi-Mg-Cr, KCl:Na-Fe-Mg, KClzLi-Cr-Be-Ag, KCI2Na-Ag-Be-Cr, KCl Nu Tl Mg Ni etc.

Combinations in case of other alkali metal halides will be easily understood from the above concrete examples, without further illustrations. Furthermore, numbers of the kinds of the metal ions other than alkali metal ion to be added will be suitably one to five, though they are particularly limited.

Thus, when writing and erasure of an information are carried out in the use of crystals having M-center which have been previously known, M-center will migrate one latice length and combine with another M-center or F- center to form other color centers during repeated writings and eliminations, by which the M-center disappeares. Contrary to this, the crystals to be obtained by this invention have few such possibility, because at least two kinds of impure materials which have a different action to each other are fixed in the crystals. Therefore, they will be usable as a photosensitive material having a long life, i.e., a reversible photosensitive mate rial. In addition, their photosensitivity itself has multiplicatively increased, as compared with that of the crystals of alkali metal halide intermixed with either alkali metal ion or other metal ion. The crystals of this invention may be cleaved to make a thin plate which can be used as a reversible ultramicrofiche, to which a graphic description is copied through a polarizer. Besides, they may be used as an optical storage element for optical memory system, the material for pattern information filing system and the like.

The following examples serve to illustrate the invention without being limited thereto.

EXAMPLES Sodium fluoride g) of a purity of 99.999 was melted in a platinum melting pot, to which a prescribed amount of each of an alkali metal fluoride and metal fluoride(s) were added and melted. Then crystals were grown and isolated by Kyropoulos method. The resulting crystals were irradiated with electron beam (2 MeV, 5 2A) for 10 mins. to form M-center, by which concentration of M-center became about l0 /cm A photosensitivity was measured by applying a light of super pressure mercury lamp to each of crystals obtained. The results are shown in the following table.

TABLE Addition Content of Compound Compound Amount Added in Crystals Sensitivity Added mol.% mol.% l0' em /mJ Pure NaF 3.0

LiF 2.0 0.28 5.7 AgF 0.5 0.0004 5.4 PbF 0.3 0.00018 l 1.2 znF 0.5 0.014 30.1 BeF 0.5 0.014 27.9 MgF 0.5 0.007 37.7 CaF 0.5 0.04 26.7 SrF 1.0 0.026 19.8 MnF 0.1 0.0005 10.8 NiF 0.5 0.0003 9.9 CoF 0.5 0.013 9.8 CrF 0.2 0.002 12.7 lnF 0.5 0.0003 4.7

LiF 0.5 0.077 AgF 1.0 0.00009 36.4 LiF 0.5 0.085 PbF 0.5 0.0025 29.6 LiF 2.0 0.107 MgF- 1.0 0.15 213 LiF 2.0 0.126 BeF 1.0 0.053 197 LiF 1.0 SrF 1.0 48.0 LiF 0.2 MgF, 0.2 125.0 CrF- 0.2

What is claimed is:

l. A dichroic reversible photosensitive material, which consists of an alkali metal halide crystal having M-centers, said crystal containing an alkali metal ion having a smaller ion radius than that of the alkali metal in said alkali metal halide and one or more metal ions which possess the ability to act as an electron trap selected from the group consisting of alkali earth metal ions and transition metal ions.

2. The material in accordance with claim 1, in which the alkali metal halide crystal is crystal of sodium fluoride, sodium chloride, sodium bromide, sodium iodide, potassium fluoride, potassium chloride, potassium bromide, lithium fluoride, rubidium bromide, cesium fluoride or cesium bromide.

3. The material in accordance with claim 1, in which the alkali metal halide crystal is sodium fluoride or sodium chloride crystal.

4. The material in accordance with claim 1, in which crystal contains said alkali metal ion and said alkali earth metal ions or a mixture of said alkali earth metal ions and transition metal ions.

5. The material in accordance with claim 1, in which the alkali earth metal ion is an ion of beryllium, magnesium, calcium, strontium or barium.

6. The material in accordance with claim 1, in which the transition metal ion is an ion of iron, cobalt, nickel, titanium, vanadium, chromium or magnesium.

7. The material in accordance with claim 1, in which the alkali metal halide crystal is sodium fluoride crystal, the alkali metal ion having smaller ion radius than that of tha alkali metal of said alkali metal halide is lithium ion and the metal ion other than alkali metal ion which possesses an ability to act as as electron trap is magnesium ion.

8. The material in accordance with claim 1, in which the alkali metal halide crystal is sodium fluoride crystal, the alkali metal ion having smaller ion radius than that of the alkali metal of said alkali metal halide is lithium ion and the metal ion other than alkali metal ion which possesses an ability to act as as electron trap is beryl lium ion.

9. The material in accordance with claim 1, in which the alkali metal halide crystal is sodium fluoride crystal, the alkali metal ion having smaller ion radius than that of the alkali metal of said alkali netal halide is lithium ion and the metal ion other than alkali metal ion which possesses an ability to act as an electron trap is strontium ion.

10. The material in accordance with claim 1, in which the alkali metal halide crystal is sodium fluoride crystal, the alkali metal ion having smaller ion radius than that of the alkali metal of said alkali metal halide is lithium ion and the metal ions other than alkali metal ion which possess an ability to act as an electron trap are magnesium ion and chromium ion. 

1. A DICHROIC REVERSIBLE PHOTOSENTATLIVE MATERIAL, WHICH CONSISTS OF AN ALKALI METAL HALIDECRYSTAL HAVING M-CENTERS, SAID CRYSTAL CONTAINING AN ALKALI METAL ION HAVING A SMALLER ION RADIUS THAN TAT OF THE ALKALI METAL IN SAID ALKALI METAL HALIDE ANS ONE OR MORE METAL IONS WHICH POSSESS THE ABIBITY T ACT AS AN ELECTRON TRAP SELECTED FROM THE GROUP CONSISTING OF ALKALI EARTH METAL IONS AND TRANSFUSION METAL IONS.
 2. The material in accordance with claim 1, in which the alkali metal halide crystal is crystal of sodium fluoride, sodium chloride, sodium bromide, sodium iodide, potassium fluoride, potassium chloride, potassium bromide, lithium fluoride, rubidium bromide, cesium fluoride or cesium bromide.
 3. The material in accordance with claim 1, in which the alkali metal halide crystal is sodium fluoride or sodium chloride crystal.
 4. The material in accordance with claim 1, in which crystal contains said alkali metal ion and said alkali earth metal ions or a mixture of said alkali earth metal ions and transition metal ions.
 5. The material in accordance with claim 1, in which the alkali earth metal ion is an ion of beryllium, magnesium, calcium, strontium or barium.
 6. The material in accordance with claim 1, in which the transition metal ion is an ion of iron, cobalt, nickel, titanium, vanadium, chromium or magnesium.
 7. The material in accordance with claim 1, in which the alkali metal halide crystal is sodium fluoride crystal, the alkali metal ion having smaller ion radius than that of tha alkali metal of said alkali metal halide is lithium ion and the metal ion other than alkali metal ion which possesses an ability to act as as electron trap is magnesium ion.
 8. The material in accordance with claim 1, in which the alkali metal halide crystal is sodium fluoride crystal, the alkali metal ion having smaller ion radius than that of the alkali metal of said alkali metal halide is lithium ion and the metal ion other than alkali metal ion which possesses an ability to act as as electron trap is beryllium ion.
 9. The material in accordance with claim 1, in which The alkali metal halide crystal is sodium fluoride crystal, the alkali metal ion having smaller ion radius than that of the alkali metal of said alkali netal halide is lithium ion and the metal ion other than alkali metal ion which possesses an ability to act as an electron trap is strontium ion.
 10. The material in accordance with claim 1, in which the alkali metal halide crystal is sodium fluoride crystal, the alkali metal ion having smaller ion radius than that of the alkali metal of said alkali metal halide is lithium ion and the metal ions other than alkali metal ion which possess an ability to act as an electron trap are magnesium ion and chromium ion. 